ROSAT X-ray sources embedded in the ρ ophiuchi cloud core

Sophie Casanova, Thierry Montmerle, Eric D. Feigelson, Philippe André

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We present a deep ROSAT Position Sensitive Proportional Counter image of the central region of the ρ Oph star-forming region. The selected area, about 35′ × 35′ in size, is rich with dense molecular cores and young stellar objects (YSOs). Fifty-five reliable X-ray sources are detected (and up to 50 more candidates may be present) above ∼1 keV, doubling the number of Einstein sources in this area. These sources are cross-identified with an updated list of 88 YSOs associated with the ρ Oph cloud core. A third of the reliable X-ray sources do not have optical counterparts on photographic plates. Most can be cross-identified with Class II and Class III infrared (IR) sources, which are embedded T Tauri stars, but three reliable X-ray sources and up to seven candidate sources are tentatively identified with Class I protostars. Eighteen reliable, and up to 20 candidate, X-ray sources are probably new cloud members. The overall detection rate of the bona fide cloud population is very high (73% for the Class II and Class III objects). The spatial distribution of the X-ray sources closely follows that of the molecular gas. The visual extinctions Av (estimated from near-IR data) of the ROSAT sources can be as high as 50 or more, confirming that most are embedded in the cloud core and are presumably very young. Using bolometric luminosities Lbol estimated from J-magnitudes, a tight correlation between Lx and Lbol is found, similar to that seen for older T Tauri stars in the Cha I cloud: Lx ∼ 10-4Lbol. A general relation Lx ∝ Lbol ∝ LJ seems to apply to all T Tauri-like YSOs. The near equality of the extinction in the IR J band and in the keV X-ray range implies that this relation is valid for the detected fluxes as well as for the dereddened fluxes. The X-ray luminosity function of the embedded sources in ρ Oph spans a range of Lx ∼ 1028.5 to ≳1031.5 ergs s-1 and is statistically indistinguishable from that of X-ray-detected visible T Tauri stars. We estimate a total X-ray luminosity Lx,Oph ≳ 6 × 1032 ergs s-1 from ≈200 X-ray sources in the cloud core, down to Lbol ∼ 0.1 L or M* ∼ 0.3 M. We discuss several consequences of in situ irradiation of molecular clouds by X-rays from embedded YSOs. These X-rays must partially ionize the inner regions of circumstellar disk coronae, possibly playing an important role in coupling magnetic fields and wind or bipolar outflows. Photon-stimulated desorption of large molecules by YSO X-rays may be partly responsible for the bright 12 μm halos seen in some molecular clouds. Because YSO X-ray emission exceeds cosmic-ray ionization as the principal source of ionization in molecular cloud cores, it may play an important role as a feedback agent in the self-regulation of star formation.

Original languageEnglish (US)
Pages (from-to)752-770
Number of pages19
JournalAstrophysical Journal
Issue number2
StatePublished - Feb 1 1995

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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